The electron‐attachment rate constant for Cl2has been measured in mixtures of N2/Cl2and Ar/Cl2at room temperature and at 250 °C in an electron‐beam‐controlled discharge. From these measurements and the use of a numerical solution of the Boltzmann equation we have been able to calibrate the relative electron‐attachment cross section of Cl2that has been measured by Tam and Wong and by Kurepa and Belic. The peak value of this cross section that gives the best fit to our data is 2.2×10−16cm2at room temperature.